The AMS detector on the International Space Station (Image: NASA)
It's been a decade in space for the Alpha Magnetic Spectrometer (AMS) - and a decade of amazing cosmic discoveries. On its final flight on 16 May 2011, space shuttle Endeavour delivered the AMS detector, which was assembled at CERN, to the International Space Station. And by 19 May 2011 the detector was installed and sending data back to Earth - to NASA in Houston and then from NASA to CERN for analysis. Ten years and more than 175 billion cosmic rays later, AMS has delivered scientific results that have changed and confounded our understanding of the origin of these particles and how they journey through space at almost the speed of light.
Cosmic rays come in many species. They are mainly the atomic nuclei of hydrogen, that is, protons, but also include the nuclei of heavier elements as well as electrons and the antimatter counterparts of protons and electrons. And they fall into two main types: primary and secondary. Primary cosmic rays are mostly produced in supernovae explosions in the Milky Way and beyond, and they can travel for millions of years before reaching AMS. Secondary cosmic rays are created in interactions between the primary cosmic rays and the interstellar medium.
AMS measures the properties of the cosmic rays that reach it to try and shed light on the origin of dark matter, antimatter and cosmic rays as well as to explore new phenomena. Highlights from the many AMS results obtained in its first ten years include a result showing that the numbers, or more precisely the "fluxes", of several types of secondary cosmic rays are all surprisingly identical to one another and very different from those of primary cosmic rays. AMS also reported an analysis of the flux of cosmic-ray positrons, the antimatter particles of electrons, indicating that at high energies these cosmic rays predominantly originate either from the annihilation of dark matter particles in space or from other cosmic sources such as fast-spinning stars called pulsars.
Other highlights include a result showing that, contrary to expectations, primary cosmic rays have at least two distinct classes, one made of light nuclei and the other made of heavy nuclei. Intriguingly, however, a more recent study revealed that iron nuclei - the most abundant primary cosmic rays after silicon nuclei and the heaviest cosmic rays measured by AMS until now - belong unexpectedly not to the same class as the other heavy nuclei but instead to the class of light nuclei.
"It's impossible to do justice to all of the AMS results, but one thing is clear," says AMS spokesperson Samuel Ting. "Over the past ten years, AMS has challenged time and again conventional theory of cosmic-ray origin and propagation, transforming our understanding of these cosmic particles."
AMS continues to collect data, following the successful completion of a series of spacewalks - unparalleled in complexity for a space intervention - that have extended its remaining lifetime to match that of the International Space Station. And if the results obtained in the past decade are anything to go by, more cosmic discoveries will no doubt be in store.
